|HONGTHANAT, NAJPHAK - Iowa State University|
|THOMPSON, MICHAEL - Iowa State University|
|RUSSELL, JAMES - Iowa State University|
|ISENHART, THOMAS - Iowa State University|
Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2016
Publication Date: 7/8/2016
Citation: Hongthanat, N., Kovar, J.L., Thompson, M.L., Russell, J.R., Isenhart, T.M. 2016. Phosphorus source – sink relationships of stream sediments in the Rathbun Lake watershed of southern Iowa, USA. Environmental Monitoring and Assessment. 188(8):453. doi: 10.1007/s10661-016-5437-6.
Interpretive Summary: Livestock grazing in pastures has been identified as a possible factor contributing to sediment and phosphorus (P) delivery to Rathbun Lake and associated water bodies in southern Iowa. We monitored stream water and collected stream bed and bank sediments from four representative sites within the 143,000-ha watershed, and estimated the risk of P loss to the stream water column, using three P loss indices. In general, most of selected sediments had low risk of P loss as indicated by soil test P via Mehlich-3 extraction (PM3) and degree of P saturation (DPS). However, equilibrium phosphorus concentration (EPC) values, which provide an estimate of P retention or release by soils and sediments, indicated that at one of the four sites, sediments could behave as sources by releasing P to the water during most of the year. At one of the sites, there was little risk of P loss, while at the other two sites, the risk was greater in summer months when P concentrations in stream water tended to be lower. Of note, the EPC values of the bed sediments at all of the sampling sites exceeded the proposed nutrient limit for P concentrations in streams, which suggests that changes in land use within the watershed may, at least initially, have little effect on P transport to Rathbun Lake in these streams. The results of this work will contribute useful information to cattle producers, local environmental groups, and Cooperative Extension and NRCS personnel interested in reducing the negative effects of livestock grazing on water quality.
Technical Abstract: The surface waters of Rathbun Lake watershed in southern Iowa are impacted by agricultural sources of sediments and nutrients, including phosphorus (P). Because stream sediments often play an important role in regulating P concentrations in stream water, we investigated sediment – water column P relationships in four creeks within the watershed, and then evaluated the relationship between sediment properties and indicators of the risk of P loss. Based on Mehlich-3-extractable P (17 to 68 mg kg-1) and degree of P saturation (2 to 12%), stream bank and bed sediments at the four sites were unlikely to serve as major sources of P. However, equilibrium P concentrations, which ranged from 0.02 to 0.12 mg L-1, indicated that bed sediments could release P to the water column depending on dissolved P (DP) concentrations in the stream water and the time of year. The likelihood of P desorption from the sediments increased with increasing pH (r = 0.92, p < 0.01) and sand content (r = 0.78, p < 0.05), but decreased with clay content (r = -0.72, p < 0.05) and iron (Fe) (r = -0.93, p < 0.001) associated with organic matter. From these results, we speculate that changes in land use within the riparian areas may, at least initially, have little effect on P concentrations in the streams. Low concentrations of DP relative to total P (TP) in these streams, however, suggest that P loads to Rathbun Lake can be reduced if P inputs from eroded bank sediments are controlled.